PDP (plasma display panel) is the likeliest to be chosen as a large TV display for the next generation. In “Technical Trends of PDP Materials” (written by Yasuo Katsuya, Hitachi Chemical Technical Report No. 33(7), Sep. 16, 1999), a process for fabricating PDP and major materials used in each step of the process are described in detail (non-patent document 1).
In a structure of general PDP, an electrode for plasma discharge is formed on a front side glass substrate, and on the electrode, a dielectric layer (insulator) is formed. This dielectric layer needs to have resistance to high pressure and transparency to visible light.
In a method for forming a general dielectric layer, on a front side glass substrate on which an electrode has been formed, a dielectric paste of an appropriate viscosity is uniformly applied by screen printing or by the use of a bar coater, a roll coater, a slit coater or the like, then dried to remove the solvent and fired at 500 to 600° C. to decompose a binder resin in the dielectric paste.
As the binder resin in the dielectric paste, ethyl cellulose has been exclusively used heretofore because it has excellent printing properties. However, because ethyl cellulose is a rigid resin, it has a problem in the processability such as occurrence of crazing or wrinkle in the drying step. Then, in Japanese Patent Laid-Open Publication No. 246236/1999 (patent document 1) or the like, plasticizers such as phthalic esters are added to improve processability, but the plasticizers and the solvent are different in the drying rate. Therefore, more strict temperature control is required in the drying step, and the production process becomes more complicated, that is, there is yet room for improvement.
The sealing glass paste for PDP is mainly constituted of a low-melting point glass powder, an inorganic filler, a binder resin and a solvent. In a method for sealing general PDP substrates, a sealing glass paste (also referred to as “seal-up glass paste”, “seal glass paste” or “frit glass paste”) is filled in a gap between a back side substrate and a front side substrate by means of screen printing or a dispenser, then dried to remove the solvent and fired at 400 to 500° C. to decompose the binder resin. As the binder resin, ethyl cellulose has been exclusively used heretofore because it has excellent viscosity properties. However, because ethyl cellulose is carbonized when it is thermally decomposed, the degree of thermal decomposition is liable to become insufficient to cause deterioration of a sealing glass, resulting in a problem of shortening of a life of PDP.
Acrylic resins having better thermal decomposability have been studied as binders, but the resulting pastes exhibit too high flowability and insufficient viscosity though the thermal decomposition properties have been improved. Then, in for example Japanese Patent Laid-Open Publication No. 255587/2002 (patent document 2), acrylic resins improved in the viscosity properties are-proposed. These acrylic resins, however, have high molecular weight, and therefore, spinnability (thread forming) is brought about between a screen and a printed surface in the screen printing, so that there is yet room for improvement of the printing properties.
In PDP, a front side glass substrate and a back side glass substrate are generally provided in such a manner that they face each other, and the space between these glass substrates is divided by barrier ribs. The barrier ribs are generally formed by a process of sandblasting. In this process, a barrier rib material paste is applied onto a back side glass substrate by screen printing or by the use of a roll coater, a slit coater or the like and then dried to remove the solvent, whereby a layer of the barrier rib material having a uniform thickness is obtained. Subsequently, a dry film resist (DFR) is laminated thereon, then exposed to light and developed. Thereafter, the barrier rib material which has not been coated with the resist is removed by sandblasting to form barrier ribs at the desired places. Other processes to form barrier ribs include a process wherein barrier ribs are directly printed on the back side glass substrate by screen printing and a process wherein a barrier rib material is applied onto the whole surface of the back side glass substrate by screen printing or by the use of a roll coater, a slit coater or the like and the paste of the barrier rib material on other areas than the areas corresponding to barrier ribs is scraped out by a comb-shaped blade to form barrier ribs.
The barrier ribs thus formed are fired at 500 to 600° C. to decompose the binder resin. As the binder resin, ethyl cellulose has been exclusively used heretofore because it has excellent printing properties. However, because ethyl cellulose is rigid, it has a problem that the paste is poor in the processability such as sandblasting property unless a plasticizer is added to the paste to decrease rigidity. By the addition of the plasticizer, the processability is improved, but there resides another problem that the barrier ribs are liable to have defects such as crazing in the drying step or the firing step under the influence of the plasticizer.
In order to improve processability, use of a mixture of ethyl cellulose and a hydroxyl group-containing acrylic resin as a binder has been proposed (Japanese Patent Laid-Open Publication No. 54992/2003 (patent document 3)). However, when a mixture of two kinds of binder resins is used, there occur problems such that the preparation process of the paste becomes more complicated and the quality control of the paste becomes more complicated, so that there is yet room for improvement.
The phosphor of PDP is formed by the use of a phosphor paste mainly comprising a phosphor powder, a binder resin and a solvent. The phosphor paste is filled between ribs by screen printing or the like, then dried to remove the solvent and fired at 400 to 500° C. to decompose the binder resin. As the binder resin, ethyl cellulose has been exclusively used heretofore because it has excellent printing properties. However, because ethyl cellulose is carbonized when it is thermally decomposed, carbon remains in the phosphor to lower luminance of fluorescence.
Acrylic resins having better thermal decomposability have been studied as binders, but the resulting pastes exhibit too high flowability and insufficient printing properties. Then, in for example Japanese Patent Laid-Open Publication No. 329256/2001 (patent document 4), acrylic resins improved in the printing properties are proposed. These acrylic resins, however, have high weight-average molecular weight of 600000 to 2000000, and therefore, spinnability is brought about between a screen and a printed surface in the screen printing, so that there is yet room for improvement of the printing properties.
On the other hand, in Japanese Patent Laid-Open Publication No. 355618/2000 (patent document 5), the present inventors disclose that a polyurethane resin that is used in the present invention has excellent thermal decomposability, but this has not led to an invention of a printing paste.
In Examples C-17 to C-19 of U.S. Pat. No. 6,646,093 (patent document-6), a cement paste comprising a thickening agent made of a polyurethane resin that is used in the present invention, cement and water is disclosed. This cement paste is prepared in the light of its characteristics that retardation of setting does not substantially occur when this thickening agent is used. The cement reacts with water (hydration reaction) in the paste to harden the cement paste, so that when the cement paste is used, a firing step to harden the paste is unnecessary. Therefore, the cement paste is different from the aforesaid dielectric paste or the like at this point.
In the patent document 5 and the patent document 6, there is no description of printing properties of a paste containing the polyurethane resin, and nothing is disclosed on that a paste containing the polyurethane resin can be favorably used for printing, more specifically, if a paste containing the polyurethane resin is used, screen printing can be carried out with rarely bringing about a spinnability of the paste and a smooth print can be obtained.
Patent document 1: Japanese Patent Laid-Open Publication No. 246236/1999
Patent document 2: Japanese Patent Laid-Open Publication No. 255587/2002
Patent document 3: Japanese Patent Laid-Open Publication No. 54992/2003
Patent document 4: Japanese Patent Laid-Open Publication No. 329256/2001
Patent document 5: Japanese Patent Laid-Open Publication No. 355618/2000
Patent document 6: U.S. Pat. No. 6,646,093
Non-patent document 1: “Technical Trends of PDP Materials” (written by Yasuo Katsuya, Hitachi Chemical Technical Report No. 33(7), Sep. 16, 1999)